1. Field of the Invention
The present invention relates to an accelerometer, and particularly to an accelerometer manufactured by applying semiconductor micro-machining technology, and a package structure for the accelerometer.
2. Description of the Related Art
In recent years, a technique for manufacturing a microscopic structure having a size of several hundred micrometers or thereabouts by use of micro-machining that applies semiconductor micro-machining technology has drawn much attention. Applications of such microscopic structures to various types of sensors, optical switches used in the field of optical communication, high-frequency components and the like have been studied. Generally, such components that apply micro-machining are manufactured by using a silicon process, so that these components can be integrated into the same chips as those of an integrated circuit of a signal processing system. As a result, a system having one certain function can be formed with only a single chip. Thus, an element having the above function is referred to as Micro Electrical Mechanical System (MEMS) or Micro System Technology (MIST).
Components utilizing MEMS include an accelerometer that is widely applied to an air bag control apparatus for an automobile, an information measuring system of an underground environment such as seismic activity or the like, an earthquake-resistant system of information communication components, and the like. Such accelerometers have been disclosed in, for example, JP-A No. 7-225240 and JP-A No. 11-248737.
FIG. 2 is a cross-sectional view showing the structure of a conventional accelerometer disclosed in JP-A No. 7-225240. This accelerometer has a sensor chip 1 that detects an acceleration. The sensor chip 1 is formed by a peripheral frame 1a, an internal mass portion 1b, and two beams 1c that elastically support the mass portion 1b on the frame 1a in a cantilevered manner. These components are integrally formed with a silicon wafer by use of semiconductor manufacturing technology. A piezoelectric resistive element (not shown) whose resistance value varies depending on an amount of deflection occurring in the beam is formed on the upper surface of the beam 1c. The piezoelectric resistive element is connected to a connection pad on the upper surface of the frame 1a. A blocking portion 2 having the shape of a rectangular frame is provided in an outer peripheral region of the frame 1a of the sensor chip 1. Further, a glass cover 3 is bonded to the lower surface of the frame 1a. 
A sensor main body comprised of the sensor chip 1, the blocking portion 2, and the cover 3 is placed on a mounting substrate 4, and thereafter, a connecting terminal of a detection circuit or the like, that is provided on the mounting substrate 4, and a connecting pad of the sensor chip 1 are connected by a bonding wire 5. Further, a resin portion 6 is formed extending from the blocking portion 2 in a manner to encapsulate the sensor chip 1 and the bonding wire 5. The blocking portion 2 is provided around the frame 1a so as to have the shape of a rectangular frame. Therefore, even when the accelerometer is molded by molding resin on the mounting substrate 4, the molding resin does not come into the frame 1a, and elastic displacement of the beams 1c or the mass portion 1b is not impeded.
In the aforementioned accelerometer, the sensor main body is protected by the resin portion 6. Therefore, an impact caused by dropping of the accelerometer is alleviated, and it is unlikely that the sensor may be broken. As a result, reliability can be raised. Further, heat distortion due to the environment, or the like is restrained by the resin portion 6, thereby improving the temperature characteristics.
However, in the aforementioned accelerometer, the sensor chip 1 is bonded to the glass cover 3, and the cover 3 is placed on the mounting substrate 4, and thereafter, these components are entirely sealed with the molding resin 6. For this reason, there exist problems that the thickness of the accelerometer becomes larger and a process for bonding the cover 3 is required, thereby increasing manufacturing costs.
On the other hand, if the sensor chip 1 is directly bonded onto the mounting substrate 4 without using the cover 3 for the purpose of cost reduction, a problem newly arises that an adhesive seeps out from the frame 1a and reaches the bottom of the mass portion 1b, thereby causing the mass portion 1b and the mounting substrate 4 to adhere to each other. If, in order to solve the aforementioned problem, a gap between the mass portion 1b and the mounting substrate 4 is made larger (for example, 50 μm), another problem arises that the amount of movement of the mass portion 1b due to an acceleration or the like may increase and the beams 1c that support the mass portion 1b may be broken.